3GPP is responsible for the development and maintenance of GSM/GPRS, WCDMA/HSPA and LTE standards. This disclosure focuses primarily on LTE, based on Orthogonal Frequency-Division Multiplexing (OFDM) and Single-Carrier Frequency-Division Multiple Access (SC-FDMA), which is also known as the Long Term Evolution of UTRAN, or Evolved Universal Terrestrial Radio Access Network (E-UTRAN). Detailed UTRAN radio access specifications are described in the 25-series of 3GPP specifications, while E-UTRAN specifications are found in the 36-series. LTE was introduced in 3GPP Release 8, but the development and future evolution of LTE continues in 3GPP Releases 9, 10 and so on.
A radio base station provides communication services in one or more areas, or cells. A cell is further associated to a carrier frequency and a radio access technology, such as GSM/GPRS, WCDMA/HSPA, LTE as well as similar and future access technologies. Mobile stations in the cell, served by the radio base station, receive from (downlink, forward link) and/or transmit to (uplink, reverse link) the radio base station at a carrier frequency. In time division duplex, TDD, the same carrier frequency is used for both uplink and downlink, while in frequency division duplex, different carriers frequencies are used, typically at a specific duplex frequency separation. In the sequel, the main description concerns, without loss of generality, downlink communication and the same can be applied for uplink as well, and either FDD or TDD or combinations can be considered.
Carrier aggregation is used in LTE, i.e. LTE-Advanced, in order to increase the bandwidth, and thereby increase the bitrates. Carrier aggregation implies that a terminal receives or transmits on multiple component carriers. In the case of carrier aggregation, multiple LTE carriers, each with a bandwidth up to 20 MHz, can be transmitted in parallel to/from the same terminal, thereby allowing for an overall wider bandwidth and correspondingly higher per-link data rates.
According to LTE release 10, up to five component carriers, possibly of different bandwidths up to 20 MHz, can be aggregated allowing for overall transmission bandwidths up to 100 MHz A terminal capable of carrier aggregation may receive or transmit simultaneously on multiple component carriers.
A terminal capable of carrier aggregation has a downlink primary component carrier and an uplink primary component carrier. In addition, it may have one or several secondary component carriers in each direction. Different terminals may have different carriers as their primary component carrier, i.e. the primary component carrier configuration is terminal specific. The fact that carrier aggregation is terminal specific, i.e. that different terminals may be configured to use different set of component carriers, is useful from a network perspective to balance the load across component carriers.
It is possible to configure a terminal to aggregate a different number of component carriers originating from the same access node, eNodeB, and of possibly different bandwidths in the uplink and the downlink. The number of downlink component carriers that can be configured depends on the downlink aggregation capability of the terminal. The number of uplink component carriers that can be configured depends on the uplink aggregation capability of the terminal. Component carriers originating from the same eNodeB need not to provide the same coverage.
When carrier aggregation is used, there are a number of serving cells, one for each component carrier. The RRC connection is only handled by one cell, the primary cell, served by the primary component carrier. The other component carriers are all referred to as secondary component carriers, serving secondary cells. The secondary component carriers are added and removed as required, while the primary component carriers are only changed at handover.
Carrier aggregation enables a wireless device or user equipment, UE, to have one or more secondary cells. Carrier aggregation is performed at MAC layer. Since carrier aggregation is performed at MAC layer, the higher layers are not aware of which cell is being used for transmission; thus, the wireless device is only logically represented in the primary cell.
All idle mode procedures apply to the primary component carrier only, i.e. carrier aggregation with additional secondary carriers configured only applies to terminals in an RRC-CONNECTED state. Upon connection to the network, the terminal performs the related procedures such as cell search and random access following the same steps as in the absence of carrier aggregation. When going from IDLE to CONNECTED mode the wireless device performs attach to the cell which the wireless device is currently camping on. The cell to which the wireless device successfully attaches is the primary cell of this wireless device. When a communication between the network and the terminal has been established, additional secondary component carriers can be configured.
For load management each cell calculates the current CONNECTED load and compares this load with that of neighbor cells. If a neighbor cell has less load than this cell, a handover or release with redirect is performed to move one or several wireless devices to the lesser loaded cell (a load management triggered action).
Different load metrics can be considered. At the Packet Data Convergence Protocol, PDCP, layer, each wireless device is assigned one or more radio bearers, which in turn are separated in signaling radio bearers and data radio bearers. One natural way to introduce load contributions is to associate each radio bearer of the wireless device with a cost and sum over all radio bearers of the wireless device to come up with a total cost of the wireless device. Such a cost or load contribution is naturally associated with the primary cell, since PDCP performance is associated to the primary cell of the wireless device. At the physical layer on the other hand it is possible to associate the wireless device to a cost in terms of consumed radio resources, or radio resource utilization per carrier. This requires detailed monitoring of resource assignments per wireless device and carrier on lower layer.
Thus, in current solutions, the wireless device load contribution on higher layer is associated to a primary cell of the wireless device, while there is no wireless device load contribution associated to one or more secondary cells of the wireless device.